Oxymorons

An oxymoron is two words that don’t belong being used together. Why is the phrase “Holy War” an oxymoron? Why is it that so many conflicts in the world today are based upon religious differences yet all major world religions condemn killing?

I believe that the phrase "Holy War" is an oxymoron because something holy is something that belongs to a religion and a war is something that religious people say they shouldn't do. They are told not to kill. So the phrase "Holy War" means a war where the people are fighting for their religion when their religion is saying not to.
War is, in all, killing so you will be going against your religious bases for your religion. That doesn't quite make sense to me and the sentence is basically being contrasted.
Now, I believe that so many wars are based on religion because, well, they think that the more they fight, the stronger their religion will be. However, that's not necessarily true. The religions could be fighting for the publicity of their religion, for more people to join, or they could be doing it to help their religion stay strong. Also, this doesn't necessarily mean that they are proud of themselves. They could, the whole time, be thinking that this is a shameful act, but if they keep it up, it might be able to help whatever their need is. So, the people could be thinking they are part of this religion by trying to help, but at the same time, they could be thinking they are not part of their religion for fighting in war. I think it all depends on their thinking of how devoted they are.